Magnetic actuator

ABSTRACT

A magnetic actuator for reciprocating motion of a compressor diaphragm, piston or the like. The actuator comprises a stator with stator poles, excitation windings, and a backing plate. Non-magnetic support arms carry arced magnet poles and actuate a pair of compressed diaphragms. With the introduction of coil current, magnet poles are attracted to the central stator poles and repulsed by outer poles. A torque about pivot results and hence movement of arms with consequential actuation of the diaphragms. Performance is improved by increasing the flux produced by the electromagnet, thereby reducing flux leakage and fringing.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic actuator. The actuator maybe used in any applications where a member, for example a diaphragm,piston or plunger mounted for reciprocating motion in a predetermineddirection is to be actuated.

Magnetic actuators are known and operate by interaction between amagnetic field and electric current flowing in one or more coils orwindings. Typically magnetic actuators include an electromagnetincorporating a fixed core and a winding associated with the core,influencing a movable armature also of soft ferromagnetic material. Thearmature is one or more permanent magnets mounted on a movable actuatormember connected to the member to be actuated, the diaphragm of adiaphragm pump, for example, with the permanent magnets influenced by anelectromagnet.

GB 1557453 shows a known moving-magnet actuator, which consists of afixed soft magnetic E-core stator assembly and two parallely magnetisedpermanent magnets arranged so that they present opposite poles towardsthe stator. The magnets are attached to a pair of independent softmagnetic lever arms which are supported by a pivot point and attached tothe compressor. A single phase coil is mounted on the core central limb,and when excited by an alternating current, the magnetic arms produce analternating torque and hence displacement.

The pair of permanent magnets are arranged so the axes of magnetizationare in opposition and the motion of the two arms are synchronous,however, this can produce excessive vibration.

The above systems have proved successful for a number of products buttheir achievable performance is limited by a number of factors and thereremain several disadvantages in terms of their manufacture. For example,large air-gaps are required for mechanical clearance due to toleranceproblems and allowance for wear of the pivot points, resulting insignificant flux leakage and the creation of stray fields. Additionallythe use of soft magnetic arms, although improving the magnetic circuitover non-magnetic arms by acting as back-iron for the magnets,introduces problems of significant leakage and stray fields along thearms due to the extension of the soft magnetic component from the magnetback to the pivot point. The interaction of these soft magnetic armswith the coil excitation field also produces a reluctance or saliencyforce which distorts the excitation force profile. Another inherentfeature of such devices is the presence of unbalanced magnetic forceswhich act in a perpendicular direction to the desired direction ofmotion due to the attraction of the magnet and the swing arm componenttowards the soft magnetic stator assembly. These forces can lead toexcessive wear on the pivot system, particularly due to the cyclicnature of the force when in operation.

With regard to the electromagnet design in these actuators, currentdesigns are typified by stators of parallel tooth and slot designs whichhave a large pole area to produce the correct torque-displacementprofile. In order to simplify lamination cross section and to allowsimple coil location the pole widths may be extended along the entiretooth length. This leads to excessive volumes of material. The materialis therefore under utilised due to low levels of flux. Additionally,this feature leads to relatively narrow slots, and in order toaccommodate sufficient copper windings whilst also accounting for‘creepage and clearance’ the slots are typically deep and narrow,leading to slot leakage flux, i.e. flux produced by the coil which doesnot travel across the slot and is not available at the working air-gapfor torque production.

SUMMARY OF THE INVENTION

The present invention seeks to improve the construction and performanceof such systems.

The present invention provides an actuator for a member mounted forreciprocating motion in a predetermined direction, the actuatorcomprising a permanent magnet assembly linked to the member for movementtherewith, the magnet assembly providing at least a pair of magneticpoles having similar or opposed pole faces adjacent one another anddirected perpendicularly from the member and an electromagnet assemblyhaving a respective pair of opposite poles located opposite so thatenergisation of the electromagnet produces movement of the permanentmagnet poles towards one or other of the electromagnet poles wherein amagnetic backing member interlinks the poles of the permanent magnet toprovide a flux return path.

In a preferred embodiment, the member to be actuated is a flexiblediaphragm. The backing member acts to minimise leakage, maintain auseful magnet working point and offer an enhanced flux return pathwhilst reducing the normal force. Preferably, the magnetic backingmember is mounted independently of the magnet and more preferably islocated in a stationary position with respect to the electromagnetassembly, parallel to the electromagnet poles to define an air space inwhich the magnets are free to oscillate. Regardless of the magnetarrangement, the backing member improves performance by increasing thecomponent of flux produced by the electromagnet assembly which interactswith the magnet in torque production so that for a given level ofexcitation a larger portion of flux will extend perpendicularly awayfrom the electromagnet pole faces across the air gap to the backingmember, whereas normally the flux would have a significant component offlux fringing, and flux not contributing to the torque production.

Preferably, the backing member comprises an axial dimension comparablewith the magnet pole face axial dimension. Conveniently, the backingmember may comprise a backing plate.

Preferably, the backing member may be held in a fixed position or morepreferably an adjustable mounting may be used to allow adjustment of theair-gap between the backing member and the magnet poles to allowadjustment of the output by altering the magnetic circuit flux and hencetorque.

In a preferred embodiment, the backing plate may be contoured with aradius or an approximation of a curve, as per the pole faces of theelectromagnet, to maintain the desired air-gap over the moving magnetstroke. Alternatively the backing plate may be straight or ‘v’-ed.

In the preferred embodiment the magnets may be arc segments (as shown inFIG. 6) which when combined with a radially contoured backing plate andpole faces, will achieve a minimum variation in air-gap and hence allowminimum mechanical clearance. The magnet segments may be radially ordiametrically magnetised. However, for many instances, rectangularmagnets may be preferred.

Preferably, the link between the magnet assembly and the diaphragmcomprises pivotable arms. More preferably, the arms support the pair ofmagnetic poles respectively.

Preferably, the magnets may be mounted such that the magnetic-air gap isnot extended.

Preferably, the arms may be non-magnetic, thereby avoiding the problemsof leakage and stray fields along the arms associated with magneticarms, due to the extension of the soft magnetic component from themagnet back to the pivot point.

The use of non-magnetic arms allows the use of injection or compressionmoulded plastics to be considered, hence allowing a reduction in movingmass, whilst maintaining the desired degree of rigidity and strength.

Preferably, the pivot, arm, and diaphragm components may be integratedinto a single assembly, which significantly reduces component count andgreatly increases reliability whilst minimising possible noise andvibration.

Further the mechanical linkage with the moulded diaphragm components mayalso be improved by ensuring an air tight seal. The use of injectionmoulded components also increases tolerance control which combined withthe reduction of the unbalanced normal force allows minimisation of themechanical clearance between magnet and electromagnet backing member andthereby minimising the magnetic air-gap, and improving the magneticcircuit efficiency.

In the preferred embodiment, the electromagnet assembly comprises an Estator including outer pole faces extending inwards from the outer teethtowards the centre pole. This configuration allows a wider shallowerslot region to house the same volume of coil and hence reduced slotleakage.

In another preferred embodiment, the electromagnet assembly may comprisepole faces located remote from the rest of the assembly. For example, byextending the length of the teeth, the coil and back-iron may be locatedexternally to a casing which houses the moving arm and magnetassemblies. Preferably, the pole faces of the stator may be moulded intothe casing ensuring a good pneumatic and acoustic seal, withoutextending the effective magnetic air-gap. In addition to improving thepneumatic and acoustic sealing arrangement, this feature also has theadvantage of placing the main source of heat i.e. the coil, remotelyfrom the plastic components of the pneumatic system whose performance isstrongly influenced by temperature. Further, the selection of coils forglobal markets with varying voltage levels, and the replacement offailed coils is more readily achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present invention will now be described with referenceto the accompanying drawings in which:

FIG. 1 shows a schematic diagram of the moving-magnet actuator accordingto a preferred embodiment of the present invention;

FIG. 2 shows the mounting of one of the permanent magnet poles;

FIG. 3 shows typical field distributions due to the permanent magnets;

FIG. 4 shows typical field distribution due to coil excitation only;

FIG. 5 shows a schematic diagram of another embodiment of the actuator.

FIG. 6 shows a schematic diagram of the moving-magnet actuator, wherethe permanent magnets are arc segments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the preferred embodiment of the actuator 1 hasthree stator poles 3 a, 3 b, 3 c and two magnet poles, 2 a, 2 b, twoindependent arm assemblies 4 a, 4 b are used to actuate a pair ofcompressor diaphragms or bellows, 5 a, 5 b. The stator consists of twosoft magnetic components, an E-core 3, which carries the excitationwinding 6 and a backing plate 7. The stator structure may be constructedof laminated or solid ferromagnetic material or a suitable mixture.

The E-core limbs or teeth direct flux to three poles at the air-gap, alarge main pole 3 b, and two side poles 3 a, 3 c displaced equally fromthe central tooth by a slot in which is housed the excitation coil 6.The excitation coil 6, located on the central tooth, is supplied by theexcitation voltage which may be mains voltage or an alternating voltagesource derived from a power supply, or an electronically commutated dcsupply. The coil may be pre-wound on a bobbin or former. The coil 6 andstator teeth 3 are arranged so that for a given direction of current inthe coil winding the centre pole 3 b will produce an excitation field ofone polarity in the air-gap region, whilst the outer poles 3 a, 3 c willproduce a field of opposite polarity.

Displaced from the first stator component is a second static member orbacking plate 7, of soft magnetic material which is used to improve thefield due to the coil 6 by acting as a return path for the flux and toimprove the flux travelling in the desired direction normal to the polesurface. The backing plate 7 has an axial dimension comparable with thepole face axial dimension and comparable width as the E-core from theoutside edge of the outer teeth. Both dimensions may be slightlyincreased to improve the backing plate's effectiveness.

In the air-space defined by the stator E-core pole pieces 3 a, 3 b, 3 cand the backing plate 7, are located the permanent magnet members 2 a, 2b, which are suspended and movable via the non-magnetic support arms 4a, 4 b. The permanent magnets 2 a, 2 b may be arcs or parallelepipedsections. Although, parallelepiped or rectangular sections may bepreferred for cost reasons, the use of arms supporting profiled polepieces and a backing plate, offers advantages by maintaining a constantair-gap over the radial motion of the magnet. When arc segments are usedboth radial magnetisation and diametric or parallel magnetisation may beconsidered. However, for the cost effective rectangular sectionsparallel magnetisation may be used. The permanent magnet material may besintered, injection moulded or compression bonded ferrites orrare-earths.

The axes of magnetisation for both magnets, is in a direction normal tothe desired motion as shown in FIG. 1, with both magnets presentingsimilar poles towards the stator. Similar poles allow anti-vibration butcould be with opposite magnetisation and have synchronised movement.

The magnets are suspended via two independent non-magnetic pivot-arms 4a, 4 b, which allow the magnets 2 a, 2 b to be displaced along thedesired excursion. The arms 4 a, 4 b are arranged to ensure the magnet 2a, 2 b position lies approximately mid-way between the stator 3 andbacking plate 7, over the entire excursion.

As shown in FIG. 2, the arms 4 a, 4 b hold the magnets 2 a, 2 b fromabove and below so as not to have additional material on the facesparallel to the E-core 3 and backing plate 7. Hence the magnetic air-gapis not increased for a given mechanical clearance. The magnets 2 a, 2 bmay be attached to the arms 4 a, 4 b with an adhesive or by anover-moulding process or mechanical location method, particularly in thecase of plastic moulded magnets which maybe moulded with features forlocation and clipping to the arms.

To facilitate the limited circular oscillations a pivot point 11 islocated at the opposite end of the arm to the magnet. The pivot 11 maybe of a flexure of plastic, or a thinning of the rigid plastic arm or ametallic insert may be employed.

During operation of the actuator, a dominant flux is produced by thepermanent magnets 2 a, 2 b, the approximate flux paths being shown inFIG. 3. The flux path due to the excitation of the coil 6 alone is shownin FIG. 4.

With no coil current present, a small saliency force is present due tothe interaction of the magnet with the salient soft-magnetic structure.The salient force displacement characteristic has an unstableequilibrium position when the magnet is approximately at the mid pointof the excursion and midway between the central pole 3 b and outer pole3 a or 3 c, and the pivot arms 4 a, 4 b are arranged such that themagnets 2 a, 2 b lie approximately at this point with zero excitation.This null-point is defined by the position of the flexible pivot points11, with no external bending forces applied and with central equilibriumof the bellows. For a given direction of coil 6 excitation current, thecentral pole-piece 3 b will produce a field of a certain polarity,whilst the outer poles 3 a, 3 c will produce a field of oppositepolarity. The magnet 2 a, 2 b will then be attracted to the centre pole3 b and repulsed from the outer pole 3 a or 3 c in its' vicinity, orvice-versa, due to distortion of the local field and will produce atorque about the pivot point 11, and hence a movement of the arm 4 a, 4b, if sufficient force is developed to overcome the stiffness of thepivot points and load force due to the bellows etc. As the magnetsdisplace towards a given pole the flux through the central limb andhence coil, increases rapidly and induces a coil back-mmf. The number ofturns on the core are selected to produce the desired coil mmf whilstaccounting for the induced coil mmf, and level of supply voltageavailable. The motion of the magnet 2 a, 2 b, is limited by themechanical load, for example the bellows 5 a, 5 b and the restoringforce of the pivot point 11. If no mechanical limitation was present,motion towards the central position motion is limited by either themagnet 2 a, 2 b ‘clashing’ or the point at which the magnet 2 a, 2 b isfully aligned with the pole 3 b, and similarly, for the outer poles 3 a,3 c. However, the actuator 1 is usually supplied with a sinusoidallyvarying voltage, and hence the torque varies sinusoidally, reversingcyclically. The force available to accelerate the arm 4 a, 4 b inertiais then determined from the interaction of the excitation force with therestoring force due to the bellows 5 a, 5 b and pivot 11, the loadforce, for example, due to gas compression and the small saliency force.The magnitudes of the forces are chosen to ensure that the armoscillates over the desired stroke. The inertia and restoring compliancemay be selected to the mechanical resonant frequency is coincident withthe supply frequency achieving maximum displacement and efficiency.

The pivoted system outlined above is preferable, as it offers a simplesupport mechanism and allows amplification of output force due toincreased leverage at the point the output force is required. Forexample, if at the approximate midpoint of the arm between magnet andpivot point the bellow/diaphragm or other load is attached, the torqueon the arm is given by

T=F×L  (Nm).

The torque may be considered as continuous about the pivot point,therefore the output force is given by,

F _(m) F _(a) L _(a)  (N)

L_(m)

L_(a)=Total length of the arm.

L_(m)=Distance to midpoint.

Therefore, the actuator is required to generate a lower force thanrequired for direct actuation, although it has to produce a largerdisplacement. However, this is not a problem, particularly in resonantactuators.

With respect to the stator a tooth geometry as shown in FIG. 1 ispreferred in which a narrow tooth is used and the addition of a largepole face is used. The cross-sectional area of the tooth may be selectedto maintain the flux density in the tooth below an acceptable level, toavoid saturation. Whilst allowing for a large slot area and minimum coilend winding a similar technique may be employed at the outer teeth. Inthe case of a solid stator the back iron and outer teeth may be formedfrom a single strip to simplify manufacture. The outer pole faces maythen be produced by bending this outer strip parallel to the air-gap,taking due account of the associated bending radii. This may be done inbending either towards or away from the central pole, towards thecentral tooth being preferred as this allows additional slot area forthe coil without extending the overall width of the device, and alsoreducing slot leakage due to the wider slot.

In order to locate the pre-wound coil 6 on the central tooth 3 b it isnecessary to split the lamination or steel, at the pole piece to allowdirect insertion of the coil. This may be done by the use of a separatepole piece mounted on the central tooth once the coil is fitted.Alternatively, the central tooth/pole piece 3 b may be a separateassembly which is attached to a C-core component which forms both theback-iron and outer poles. This separate assembly facilitates the use oflower conductivity materials at strategic points particularlysusceptible to eddy currents, i.e. the pole pieces and backing plate.

This use of lower cost magnetic materials, coupled with a E-corestructure in which the core may be split results in significantadvantages, in the placement of the coil 6 and stator 3. For example,the stator body 3 and coil 6 may be placed outside a casing 12 in whichthe magnet-arm-bellows are located, as shown in FIG. 5. The pole piecesmay be moulded into the case wall, hence ensuring a good pneumatic andacoustic seal, whilst not extending the effective air-gap. The mainsource of heat, i.e. the excitation coil, is then placed remotely fromthe temperature sensitive pneumatic components and permanent magnets.Further, the use of a removable coil and centre tooth 3 b structureallows the replacement of failed coils and the retro-fitting ofdifferent coils required for different global markets.

What is claim is:
 1. A magnetic actuator for actuating a member mountedto provide a reciprocating motion in a predetermined direction, saidmagnetic actuator comprising: a permanent magnet assembly for providingat least a pair of permanent magnetic poles having a similar or opposedpole faces adjacent one another and directed perpendicularly withrespect to said member; an electromagnet having a respective pair ofpoles located opposite the permanent magnetic poles such that saidelectromagnet produces movement of the permanent magnetic poles towardseach other when said electromagnet is energized; and a magnetic backingmember for interlinking the permanent magnetic poles to provide a fluxreturn path; wherein said magnetic backing member is mountedindependently of said magnet assembly.
 2. The magnetic actuator asclaimed in claim 1, wherein said backing member is located in astationary position with respect to said electromagnet and parallel tothe electromagnet poles to define an air gap in which the permanentmagnetic poles are free to oscillate.
 3. The magnetic actuator asclaimed in claim 2, wherein said backing member is held in a fixedposition or, alternatively, an adjustable mounting is used to allowadjustment of the air-gap between said backing member and the permanentmagnetic poles to allow adjustment of the output by altering magneticcircuit flux and torque.
 4. The magnetic actuator as claimed in claim 1,wherein said backing member comprises an axial dimension that issubstantially similar to the permanent magnetic pole face axialdimension.
 5. The magnetic actuator as claimed in claim 1, wherein saidbacking member comprises a backing plate.
 6. The magnetic actuator asclaimed in claim 5, wherein said backing plate is selectively contouredwith a radius or an approximation of a curve, as per the pole faces ofsaid electromagnet, to maintain a desired air-gap over a moving magnetstroke.
 7. The magnetic actuator as claimed in claim 5, wherein saidbacking plate is selected from straight and ‘v’-ed.
 8. The magneticactuator as claimed in claim 5, wherein said backing plate isselectively contoured with a radius or an approximation of a curve, asper the pole faces of said electromagnet, to maintain a desired air-gapover a moving magnet stroke.
 9. The magnetic actuator as claimed inclaim 1, wherein said member is a flexible diaphragm.
 10. The magneticactuator as claimed in claim 9, wherein a link between said permanentmagnet assembly and said diaphragm comprises pivotable arms, each armsupporting one of the pair of said permanent magnetic polesrespectively.
 11. The magnetic actuator as claimed in claim 10, whereinsaid permanent magnet assembly is mounted such that a magnetic-air gapis not extended.
 12. The magnetic actuator as claimed in claim 10,wherein the arms are non-magnetic.
 13. The magnetic actuator as claimedin claim 10, wherein the arms and diaphragm are integrated into a singleassembly.
 14. The magnetic actuator as claimed in claim 1, wherein saidpermanent magnet assembly comprises magnets of arc segments which areselectively radially or diametrically magnetized.
 15. The magneticactuator as claimed in claim 1, wherein said electromagnet comprises anE stator including outer pole faces extending inwards from outer teethtowards a center pole.
 16. The magnetic actuator as claimed in claim 15,wherein said electromagnet comprises pole faces located remotetherefrom.
 17. The magnetic actuator as claimed in claim 16, wherein theouter pole faces of the stator are molded into a casing ensuring a goodpneumatic and acoustic seal, without extending an effective magneticair-gap.
 18. A magnetic actuator for actuating a member mounted toprovide a reciprocating motion in a predetermined direction, saidmagnetic actuator comprising: a permanent magnet assembly for providingat least a pair of permanent magnetic poles having a similar or opposedpole faces adjacent one another and directed perpendicularly withrespect to said member; an electromagnet having a respective pair ofpoles located opposite the permanent magnetic poles such that saidelectromagnet produces movement of the permanent magnetic poles towardseach other when said electromagnet is energized; and a magnetic backingmember for interlinking the permanent magnetic poles to provide a fluxreturn path; wherein said magnetic backing member is located in astationary position with respect to said electromagnet and parallel tothe electromagnet poles to define an air gap in which the permanentmagnetic poles are free to oscillate.
 19. The magnetic actuator asclaimed in claim 18, wherein said backing member comprises an axialdimension that is substantially similar to the permanent magnetic poleface axial dimension.
 20. The magnetic actuator as claimed in claim 18,wherein said backing member is held in a fixed position or,alternatively, an adjustable mounting is used to allow adjustment of theair-gap between said backing member and the permanent magnetic poles toallow adjustment of the output by altering magnetic circuit flux andtorque.
 21. The magnetic actuator as claimed in claim 18, wherein saidbacking member comprises a backing plate.
 22. The magnetic actuator asclaimed in claim 21, wherein said backing plate is selected fromstraight and ‘v’-ed.
 23. The magnetic actuator as claimed in claim 18,wherein said member is a flexible diaphragm.
 24. The magnetic actuatoras claimed in claim 23, wherein a link between said permanent magnetassembly and said diaphragm comprises pivotable arms, each armsupporting one of the pair of said permanent magnetic polesrespectively.
 25. The magnetic actuator as claimed in claim 24, whereinsaid permanent magnet assembly is mounted such that a magnetic-air gapis not extended.
 26. The magnetic actuator as claimed in claim 24,wherein the arms are non-magnetic.
 27. The magnetic actuator as claimedin claim 24, wherein the arms and diaphragm are integrated into a singleassembly.
 28. The magnetic actuator as claimed in claim 18, wherein saidpermanent magnet assembly comprises magnets of arc segments which areselectively radially or diametrically magnetized.
 29. The magneticactuator as claimed in claim 18, wherein said electromagnet assemblycomprises an E stator including outer pole faces extending inwards fromouter teeth towards a center pole.
 30. The magnetic actuator as claimedin claim 29, wherein said electromagnet assembly comprises pole faceslocated remote therefrom.
 31. The magnetic actuator as claimed in claim30, wherein the outer pole faces of the stator are molded into a casingensuring a good pneumatic and acoustic seal, without extending aneffective magnetic air-gap.
 32. The magnetic actuator as claimed inclaim 18, wherein said magnetic backing member is mounted independentlyof said magnet assembly.